1. Field of the Invention
The present invention relates to a film-formation method and apparatus for a semiconductor process, which are used for forming a film on a target substrate, such as a semiconductor wafer. The term “semiconductor process” used herein includes various kinds of processes which are performed to manufacture a semiconductor device or a structure having wiring layers, electrodes, and the like to be connected to a semiconductor device, on a target substrate, such as a semiconductor wafer or a glass substrate used for an LCD (Liquid Crystal Display) or FPD (Flat Panel Display), by forming semiconductor layers, insulating layers, and conductive layers in predetermined patterns on the target substrate.
2. Description of the Related Art
Conventionally, for example, poly-silicon is used for gate electrodes of transistors. Gate electrodes of poly-silicon can be easily depleted, when supplied with a bias voltage. As the thickness of gate insulating films decreases, this becomes prominent, which is one of the causes of device properties being deteriorated. In order to solve this problem, it has been studied to employ silicon germanium, which has a higher dopant activation rate, in place of silicon. For example, Jpn. Pat. Appln. KOKAI Publication No. 2003-77845 (patent publication 1: see FIG. 1 and Paragraph 17) discloses a method for forming a silicon germanium film on the surface of a semiconductor wafer. This method employs a vertical heat-processing apparatus, which supplies mono-silane (SiH4) gas and mono-germane (GeH4) gas to form a silicon germanium film by CVD.
Where a gate insulating film is formed of a silicon oxide film, and a silicon germanium film is to be formed on the surface of the silicon oxide film, film-formation hardly occurs at the initial stage. The silicon germanium film, however, shows a sudden deposition after a certain time, using germanium sticking points as footholds. This makes it difficult to provide the silicon germanium film with a uniform thickness. In order to solve this problem, a technique is known in which a seed film of amorphous silicon or poly-silicon is formed to have a small thickness of, e.g., 5 nm to 10 nm on a silicon oxide film, and then a silicon germanium film is formed on the seed film.
After the silicon germanium film is formed, the silicon germanium film is doped with an impurity element, such as phosphorous or boron. Then, a cobalt film is formed on the surface of the silicon germanium film by sputtering, and is heat-processed to form a silicide (i.e., a silicon-metal compound is produced). At this time, if the cobalt film is directly formed on the silicon germanium film, germanium is taken into the silicide film and deteriorates the properties of the transistor. For this reason, a cap film consisting of a poly-silicon film or amorphous silicon film is formed on the surface of the silicon germanium film, and then the cobalt film is formed on the cap film.